1. Field of Art
This invention relates to a method for producing high strength, high modulus mesophase-pitch-based carbon fibers. More particularly, it relates to a method of graphitization of mesophase-pitch-based carbon fibers for producing high strength, high modulus carbon fibers having excellent qualities, especially high grade of mechanical property, at relatively inexpensive cost by a stabilized operation. This invention is directed to a preferable method relating to high strength, high modulus carbon fibers having a modulus of elasticity of 75 tonf/mm.sup.2 or more and a tensile strength of 250 kgf/mm.sup.2 or more.
2. Prior Art
It is well known that petroleum pitch based carbon fibers have been heretofore produced from residual carbonaceous materials obtained as by-product of thermal catalytic cracking (FCC) of vacuum gas oil or thermal cracking of naphtha.
Carbon fibers have been used in broad application fields such as aeronautic and space construction materials and articles for the use of sports, etc., because of their various superior properties such as mechanical, chemical and electric properties, together with their advantage of light weight.
Particularly, mesophase-pitch-based carbon fibers as different from the carbon fibers produced from organic polymer-based fibers such as PAN, provide easily high modulus of elasticity by carbonization-graphitization treatment, hence demand for the production of high modulus carbon fibers having a modulus of elasticity of 75 tonf/mm.sub.2 or more is increasing. However, even in case of mesophase-pitch-based carbon fibers, a high temperature graphitization treatment is necessary in order to obtain a high modulus of elasticity. As an apparatus for obtaining high temperature, a graphitization furnace, in which a furnace element is made of a carbon material, is commonly used. For producing carbon fibers having a modulus of elasticity of 75 tonf/mm.sup.2 or more, a treatment temperature approaches to the sublimation temperature of carbon of 3000.degree. C. and there is a problem in the point that life of a furnace element is extremely short and the cost of carbon fibers becomes very expensive. Since mesophase-pitch-based carbon fibers are of high modulus of elasticity, they are brittle materials having an elongation of 0.5% or lower. Thus it is also another problem that if a forcible stretching is applied during the graphitization, bad effects occur to processability and quality of products such as forming of fluffs, etc.
Different from organic-polymer-based carbon fibers such as PAN or the like, since mesophase-pitch-based carbon fibers can provide easily relatively high modulus of elasticity, they are not usually stretched positively in the carbonization and graphitization treatment.
It is an object of the present invention to provide a process for producing carbon fibers in which the problem of occurrence of a large number of fluffs by a forcible stretching and the other problem of increasing production cost by extremely shortening the life of a furnace element of a graphitization furnace in the attempt for obtaining excessively high temperature, have been overcome, based upon the finding that an application of stretching is very effective for increasing modulus of elasticity of mesophase-pitch-based carbon fibers at the time of the graphitization at a temperature of 2600.degree. C. or more for the production of high strength, high modulus carbon fibers having a modulus of elasticity of 75 tonf/mm.sup.2 or more and a tensile strength of 250 kgf/mm.sup.2 or more.